SOLUTION EQUILIBRIA FOR URANIUM ORE PROCESSING: THE BASO4-H2SO4-H2O SYSTEM AND THE RASO4-H2SO4-H2O SYSTEM - THE FESO4-H2SO4-H2O SYSTEM

Abstract

A radio-tracer determination of the solubility of BaSO4 in H2SO4-H2O mixtures was undertaken at temperatures of 25°C and 60°C. These were subsequently modelled using the Pitzer model for the activity coefficients, and estimates were made for Ba2+-SO42- and Ba2+-HSO4- interaction parameters. The final model yields an excellent description of the solubility of BaSO4 in H2SO4-H2O mixtures up to 6 mol/kg and at both 25°C and 60°C. The logarithm of the thermodynamic solubility product constant (logKsp) for barite obtained by regression analysis was -10.02 +/- 0.02 at 25°C and -9.68 +/- 0.01 at 60°C. The values of the ion association constant derived from these data are 2.49 +/- 0.03 at 25°C and 2.55 +/- 0.1 at 60°C. The enthalpy of dissolution for BaSO4 was estimated to be 17.6 +/- 0.2 kJ mol-1.Literature solubility data for the RaSO4-H2SO4-H2O system have also been interpreted, using the Pitzer model for the necessary activity coefficients. The logarithm of the thermodynamic solubility product constant (logKsp) for radium sulfate obtained was -10.21 +/- 0.06 at 25°C, while the value of the ion association constant derived is 2.76 +/- 0.05 at 25°C. The high negative value of the Pitzer parameter β2 obtained by regression analysis of these data suggests that the explicit recognition of a RaSO4(aq) ion association species is to be preferred in this case.